It has long been a common practice in the well art to suspend an inner pipe, typically a casing string, concentrically within an outer member, typically an outer casing string or a wellhead member, by means of a hanger comprising a hanger member connected to the inner pipe and having a downwardly directed shoulder which engages an upwardly directed shoulder on the outer member as the inner pipe is run in. As the art developed, it became necessary to minimize the annular space between the inner and outer hanger members, and prior art workers have developed hangers employing a retractable hanger device carried by a mandrel on the inner pipe and capable of expanding into engagement with an outer hanger member when, as the inner pipe is run in, the mandrel reaches the outer hanger member. Pipe hangers of this type have become particularly important with the advent of offshore practices in which the hanger is located at the mudline and the outer pipe above the wellhead is of the same diameter as the outer casing below the wellhead and the annular space available for the hanger is relatively small. Such hangers are disclosed, for example, in the following U.S. patents: Nos.
3,420,308 Putch PA1 3,472,530 Fowler PA1 3,741,589 Herd et al PA1 3,893,717 Nelson
Though hangers of this general type have achieved considerable success, they still present problems which increase in severity as the annular space available at the hanger decreases and the weight of the pipe string to be supported increases. Thus, it has been difficult to assure that the retractable hanger device, which must retract radially as the mandrel passes into the outer hanger member, will expand into proper engagement with the outer hanger member. Further, with the outer hanger member provided with grooves to accommodate the retractable hanger device, engagement between the outer member and the retractable device has not been adequate to assure that the large tension loads applied by the inner pipe string are adequately supported. Both problem areas tend to require structures which are unduly large in radial directions, unduly complex and excessively expensive.
These problem areas have been addressed by the inventions set forth in the above-identified applications in which, among other things, a split locking ring is utilized which has a relatively thick main body portion and a thinner, flexible skirt depending therefrom, the relatively thick main body having a locking rib and a catching rib received in respective grooves in the outer hanger member. While provision of a thin, flexible skirt on the locking ring is advantageous, thinness and flexibility of the skirt raises the problem that, as the locking ring is run down to the outer hanger member, the skirt might catch on a shoulder, a seal or some other discontinuity in the wall of the bore through which the locking ring must travel. Should the locking ring catch on a discontinuity in the bore, the thin wall of the skirt might be bent outwardly, ruining the locking ring and, e.g., damaging seals and the like in the bore. This problem is worsened by the fact that, once landed on the outer hanger member, the combination of the inner member and locking ring must be capable of being retrieved simply by pulling upwardly on the string of pipe being suspended. The desirable thinness and flexibility of the skirt of the locking ring increases the possibility that the skirt may be deformed unduly during such retrieval. Thus, despite the advantages offered by the inventions in the above-identified applications, there has been a continuing need for improvement.